Post-operative recovery of cognitive function is an important outcome measure following surgical anaesthesia. This study examined the influence of task complexity in 12 patients undergoing elective day-case surgery using reaction time tasks that differed in their stimulus-response mapping rules (compatible, incompatible, working memory). Tasks were administered at baseline, and at 1, 4 and 24 hours post-surgery. Compared to a reference group of 18 non-anaesthetised volunteers, the patient group showed considerable slowing one hour post-operatively. This slowing remained at the time of discharge (4-hours), was absent the next day, and did not vary as a function of task complexity. Only attentional lapses in the working memory task were sensitive to the recovery process and residual impairment remained the next day. The findings indicate that tasks requiring efficient access to working memory are impaired for longer than tasks simply requiring access to information passively retained in short-term memory.